What kind of devices did you use? Was that cellular connection?
We are struggling with the same problem - a telco wants Peplink devices to transmit QinQ. Unfortunatelly the frame size is 1510 and our carriers support max. 1500.
Did you have to deal with the same issue?

Layer 2 SpeedFusion tunnels will allow QinQ traffics pass-through the tunnels by ignoring the MTU limit for the WAN. Packets will be fragmented before sending out via WAN by following the WAN allowed MTU.

Our current project requires Peplink devices to be able to tag/untag a frame. We plan to use the L2SF to carry QinQ traffic and remove the S-tag on our CPE and transmit the C-Tag to the Customers (ISP) CPE.

There are a few other ways to do it but we wanted to use Peplink-only devices in this project.